add tooling for signing

There's tools for key generation and conversion and there's tools for
signing and verifying a tree leaf. Note that the leaf signing tools
use the yet to be decided about SSH signing format, with message (ie
signers checksum) being hashed with SHA-512 to match SSH
tooling (ssh-keygen -Y).

1 files changed, 61 insertions, 0 deletions

diff --git a/tools/sigsum-sign-leaf.py b/tools/sigsum-sign-leaf.py
new file mode 100755
index 0000000..19d7f5c
--- /dev/null
+++ b/tools/sigsum-sign-leaf.py
@@ -0,0 +1,61 @@
+#! /usr/bin/env python3
+
+# Input: skeyfile shard_hint [checksum]
+# Output: tree_leaf signature
+# Example: echo foo | ./sigsum-sign-leaf.py nacl.sk 1633039200
+# be70f92465c27bf412008f26fa953d06899c53fa9867f40d9c0a1d657b188c9631699954728c719cf6b3819c1343c6e9e454cd9d519a9bf96dad3cf4cd959c0a
+
+import struct, sys, binascii
+from base64 import b64encode
+from nacl.signing import VerifyKey, SigningKey
+from nacl.encoding import HexEncoder
+from libsigntools import checksum_stdin, ssh_to_sign
+
+alg = 'sha512'
+
+def ssh_blob(vk, sig, namespace):
+    vkdata = struct.pack('!I11sI32s',
+                         11, bytes('ssh-ed25519', 'ascii'),
+                         32, vk.encode())
+    assert(len(vkdata) == 51)
+
+    assert(len(sig) == 64)
+    sigdata = struct.pack('!I11sI64s',
+                          11, bytes('ssh-ed25519', 'ascii'),
+                          64, sig)
+    assert(len(sigdata) == 83)
+
+    s = "-----BEGIN SSH SIGNATURE-----\n"
+    b = b64encode(struct.pack('!6sII51sI{}sII6sI83s'.format(len(namespace)),
+                              b'SSHSIG',
+                              1,
+                              51, vkdata,
+                              len(namespace), bytes(namespace, 'ascii'),
+                              0,
+                              6, bytes(alg, 'ascii'),
+                              83, sigdata)).decode('ascii')
+    while len(b) > 0:
+        s += b[:72] + '\n'
+        b = b[72:]
+    s += "-----END SSH SIGNATURE-----\n"
+    return s
+
+def main():
+    keyfile = sys.argv[1]
+    shard_hint = int(sys.argv[2])
+    if len(sys.argv) > 3:
+        checksum = bytes.fromhex(sys.argv[3])
+    else:
+        checksum = checksum_stdin(hashalg=alg)
+
+    with open(keyfile, 'r') as f:
+        signing_key = SigningKey(f.readline().strip(), encoder=HexEncoder)
+    namespace = 'tree_leaf:v0:{}@sigsum.org'.format(shard_hint)
+    signature = signing_key.sign(ssh_to_sign(namespace, alg, checksum)).signature
+
+    print(binascii.hexlify(signature).decode('ascii'))
+    if False:
+        print(ssh_blob(signing_key.verify_key, signature, namespace))
+
+if __name__ == '__main__':
+    main()